What does this equation mean?

When any of the coefficients $\beta_{jk}$ is nonzero, the vacuum of the a -decomposition is a many-particle state of the $\tilde a$ -decomposition, and vice versa: “how many particles are present” is a question whose answer depends on which set of modes was declared fundamental, not on the field configuration by itself. Fulling showed this could happen already in flat, static coordinates on two-dimensional Minkowski space, purely from a nonstandard but perfectly legitimate choice of time coordinate [ 2 ] . Davies showed the same mixing follows Hawking’s black-hole derivation into the Rindler wedge of ordinary flat spacetime, associating a temperature with the horizon an accelerated observer drags behind them [ 3 ] . Unruh completed the argument with a model of the measuring device itself, not just the modes: a idealized two-level system, linearly coupled to the field along its own worldline, responds to whatever the field is doing along that specific worldline, and along a uniformly accelerated worldline in the ordinary vacuum it responds exactly as though bathed in real thermal radiation at temperature $k_B$ $T_U$ = $\hbar$ a / (2$\pi$ c) [ 1 ] . None of this is proposed here. All of it is the baseline the rest of this article stands on.